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The first large scale application of ultrasound was around World War II. Sonar systems were being built and used to navigate submarines. It was realized that the high intensity ultrasound waves that they were using were heating and killing fish.[3] This led to research in tissue heating and healing effects. Since the 1940s, ultrasound has been used by physical and occupational therapists for therapeutic effects.

Ultrasound is applied using a transducer or applicator that is in direct contact with the patient's skin. Gel is used on all surfaces of the head to reduce friction and assist transmission of the ultrasonic waves. Therapeutic ultrasound in physical therapy is alternating compression and rarefaction of sound waves with a frequency of >20,000 cycles/second. Therapeutic ultrasound frequency used is 0.7 to 3.3 MHz. Maximum energy absorption in soft tissue occurs from 2 to 5 cm. Intensity decreases as the waves penetrate deeper. They are absorbed primarily by connective tissue: ligaments, tendons, and fascia (and also by scar tissue).[4]

There are three primary benefits to ultrasound. The first is the speeding up of the healing process from the increase in blood flow in the treated area. The second is the decrease in pain from the reduction of swelling and edema. The third is the gentle massage of muscles tendons and/ or ligaments in the treated area because no strain is added and any scar tissue is softened. These three benefits are achieved by two main effects of therapeutic ultrasound. The two types of effects are: thermal and non thermal effects. Thermal effects are due to the absorption of the sound waves. Non thermal effects are from cavitation, microstreaming and acoustic streaming.[1]

Cavitational effects result from the vibration of the tissue causing microscopic bubbles to form, which transmit the vibrations in a way that directly stimulates cell membranes. This physical stimulation appears to enhance the cell-repair effects of the inflammatory response.[citation needed]

Relatively high power ultrasound can break up stony deposits or tissue, accelerate the effect of drugs in a targeted area, assist in the measurement of the elastic properties of tissue, and can be used to sort cells or small particles for research.

Focused high-energy ultrasound pulses can be used to break calculi such as kidney stones and gallstones into fragments small enough to be passed from the body without undue difficulty, a process known as lithotripsy.

Ultrasound can ablate tumors or other tissue non-invasively. This is accomplished using a technique known as High Intensity Focused Ultrasound (HIFU), also called focused ultrasound surgery (FUS surgery). This procedure uses generally lower frequencies than medical diagnostic ultrasound (250–2000 kHz), but significantly higher time-averaged intensities. The treatment is often guided by Magnetic Resonance Imaging (MRI); the combination is then referred to as Magnetic resonance-guided focused ultrasound (MRgFUS).

Delivering chemotherapy to brain cancer cells and various drugs to other tissues is called acoustic targeted drug delivery (ATDD).[7] These procedures generally use high frequency ultrasound (1–10 MHz) and a range of intensities (0–20 W/cm2). The acoustic energy is focused on the tissue of interest to agitate its matrix and make it more permeable for therapeutic drugs.[8][9]

Ultrasound has been used to trigger the release of anti-cancer drugs from delivery vectors including liposomes, polymeric microspheres and self-assembled polymeric.[1]

Using ultrasound to generate cellular effects in soft tissue. This particular application has fallen out of favor as research has shown a lack of efficacy[10] and a lack of scientific basis for proposed biophysical effects.[11]

Additional physiological effects of low-intensity ultrasound have recently been discovered, e.g. the ability to stimulate bone-growth and its potential to disrupt the blood–brain barrier for drug delivery.[13]

Long-duration therapeutic ultrasound called Sustained Acoustic Medicine is a daily slow-release therapy that can be applied to increase local circulation and accelerate healing of musculoskeletal tissues after an injury.[16]